1. Field
The present disclosure relates to a system and method for controlling a concentration of a fuel supplied to a liquid fuel cell on the basis of amplitude formed by a fluctuating voltage of a fuel cell stack without using a concentration sensor, and a fuel cell apparatus using the same.
Description About National Support Research and Development
This study was supported by the Ministry of Trade, Industry and Energy, Republic of Korea (Korea Institute of Energy Technology Evaluation and Planning, Project No. 1415132261) under the superintendence of Korea Institute of Science and Technology.
2. Description of the Related Art
One of most serious problems that humankind of the twenty-first century is confronted with is limitation of energy resources and global environmental pollution. As an alternative to solve such a problem, a fuel cell is spotlighted as a next-generation energy source for solving energy exhaustion and environmental problems simultaneously. Fuel cells are greatly classified into a molten carbonate fuel cell (MCFC), a solid oxide fuel cell (SOFC), a polymer electrolyte membrane fuel cell (PEMFC) or the like, depending on kinds and characteristics of electrolyte. Among them, a direct methanol fuel cell (DMFC) is intensively studied as a power source of a small or portable device or as an alternative of a battery since it uses a liquid fuel and is convenient in supplying and storing its fuel.
In a liquid fuel cell having the direct methanol fuel cell, it is important to constantly supply a fuel at an optimal concentration and reuse a non-reacted fuel discharging from a fuel cell stack to enhance performance and energy efficiency of the fuel cell.
In this regard, as a representative fuel concentration control method used in the art, there is a control method using a concentration sensor (for example, a methanol concentration sensor). In this control method, a concentration sensor is used to measure a fuel concentration in a fuel cell circulating system, and an amount of injected crude fuel required for constantly maintaining a fuel concentration is calculated while observing a concentration variation and is supplied to the fuel cell, thereby controlling a concentration of a fuel supplied to the stack.
FIG. 1 is a schematic view showing an existing fuel cell apparatus having a concentration sensor.
As shown in FIG. 1, in the fuel cell system using a concentration sensor, a diluted fuel is supplied from a fuel mixer 1 through a fuel circulating pump 2 to the anode inlet of a fuel cell stack 3.
In addition, in the fuel cell stack 3, air is supplied from an air supply blower 4 to the cathode inlet of the fuel cell stack. The fuel mixer 1 receives a crude fuel from a crude fuel tank 5 through a crude fuel supply pump 6.
The crude fuel supply pump 6 is connected to a concentration controller 9, and the crude fuel supply pump 6 is operated according to a signal generated by the concentration controller 9 of the crude fuel supply pump. In addition, a fuel concentration sensor 17 is connected to the fuel mixer 1, and the diluted fuel stored in the fuel mixer 1 is supplied to the concentration sensor 17 by a sensor pump 16, which supplies a fuel to the concentration sensor, thereby measuring a concentration of the diluted fuel.
However, this method requires a concentration sensor and a pump attached to the sensor and thus has disadvantages of reduced energy efficiency of the fuel cell system caused by increased power consumption and an increased system volume. Further, concentration sensors presently put in the market are expensive and have a great measurement error and short life cycle. Thus, if such concentration sensors are applied to a fuel cell system, a manufacture cost for the fuel cell system increases, and safety of the fuel cell system deteriorates, which makes it difficult to maintain competitiveness in price and quality as a portable or small power source.
In order to solve the above problem, a concentration control method not using a concentration sensor has been developed. For example, there have been developed a method for additionally supplying a crude fuel as much as an amount consumed by a fuel cell (Patent Literature 1), a method for controlling an operation temperature (T) of a fuel cell stack by controlling a supply rate of the crude fuel and a cooling rate of a heat exchanger simultaneously (Patent Literature 2), a method for estimating a concentration of a liquid fuel by arbitrarily varying a fuel cell output current density and monitoring a result variation pattern of an output voltage (Patent Literature 3) or the like.
However, the above concentration control methods not using a sensor have a drawback in that a long time is needed for building a fuel consumption database required for the control method. Also, since the above concentration control methods are focused in simply maintaining a fuel concentration constantly, it is impossible to actively control a fuel concentration according to an external temperature variation. In addition, when an output current density is varied, an operation temperature (T) of the fuel cell stack, an output voltage and a fuel concentration are continuously varied, and when the performance of the fuel cell deteriorates due to a long time use, the output voltage decreases. For these reasons, an error increases in the concentration control.